1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus for forming a large number of images repetitively in an electrophotographic method and an electrostatic recording method and, in particular, to such image forming method and apparatus for developing an electrostatic latent image formed on an image carrier using a two-component magnetic developing agent consisting mainly of magnetic carriers and toners.
2. Description of the Related Art
In conventional electrophotographic method and electrostatic recording method, the entire area of the surface of a photoconductive photosensitive body is charged uniformly and is then exposed to thereby form an electrostatic latent image on the photosensitive body. After then, the electrostatic latent image is developed by a developing apparatus including a developing roller rotatable and having power of magnetic suction according to a two-component developing method for developing an electrostatic latent image using toners including coloring agents and magnetic carriers. In this development, the toners including coloring agents, due to the rotation of the developing roller, are rubbed against the magnetic carriers to be thereby electrically charged with desired values. The developed toner image is transferred onto a recording medium such as a recording sheet. The toner image on the recording medium is fixed to thereby form a recorded image. On the other hand, the remaining toners, which are not transferred onto the recording medium but are left on the photosensitive body, are cleaned by a cleaner. The above process is repeated for a long period of time. The image forming technology using the two-component developing method is proposed, for example, in Japanese Patent No. 3008838 and JP-B-6-29979.
Now, description will be given below in more detail of a developing system in which development is carried out while supplying toners to a photosensitive body 1. Firstly, description will be given of a forward-rotation developing system according to the prior art with reference to FIG. 2. A developing roller 61, which can be rotated in the same direction as the photosensitive body 1, includes a magnet 51 in the interior thereof; due to the magnetic suction power of the magnet 51, a two-component magnetic developing agent consisting mainly of toners and magnetic carriers is attracted to the developing roller 61, is then delivered due to the rotation of the developing roller 61 and is thus contacted with the photosensitive body 1, thereby developing an electrostatic latent image. The advantage of this developing system is as follows: that is, since, in the developing area where the photosensitive body and toners are contacted with each other and an electrostatic latent image is developed, the toners on the developing roller moves in the opposite direction to the moving direction of the photosensitive body, the force for the toners to rub against the surface of the photosensitive body increases and thus the cleaning effect in cleaning the surface of the photosensitive body can be enhanced. Also, because the developing roller moves in the opposite direction to the moving direction of the photosensitive body, a speed difference between the photosensitive body and developing roller is increased; and, therefore, even in case where the number of rotations of the developing roller is set small, there can be realized a high image density. However, the present developing system has a disadvantage that, since the contact between the photosensitive body and toners is strong, the quality of the image formed can be put into disorder. Also, there arises another problem that the leading end of a gang black image portion can be chipped.
Next, description will be given below of a reverse-rotation developing system according to the prior art with reference to FIG. 3. A developing roller 61, which can be rotated in the opposite direction to the photosensitive body 1, includes a magnet 51 in the interior thereof; due to the magnetic suction power of the magnet 51, a two-component magnetic developing agent consisting mainly of toners and magnetic carriers is attracted to the developing roller 61, is then delivered due to the rotation of the developing roller 61 and is thus contacted with the photosensitive body 1, thereby developing an electrostatic latent image. The advantage of this developing system is as follows: that is, since, in the developing area where the photosensitive body and toners are contacted with each other and an electrostatic latent image is developed, the toners on the developing roller moves in the same direction as the moving direction of the photosensitive body, the force for the toners to rub against the surface of the photosensitive body decreases, so that the quality of the image formed cannot be degraded but a high image quality can be realized. However, the present reverse-rotation developing system also has a disadvantage that, since the force for the toners to rub against the surface of the photosensitive body is weak, the cleaning performance of the surface of the photosensitive body is poor. Also, because the developing roller moves in the same direction as the moving direction of the photosensitive body, the speed difference between the photosensitive body and developing roller is small; and, therefore, to realize a high image density, the number of rotations of the developing roller must be set large. Further, in the case of the image quality, there is a problem that the rear end of a gang black image can be chipped. This phenomenon occurs very often especially when the image density is low.
In order to solve the above problems, there is proposed a center feed developing system including two developing rollers 61, 62 which, as shown in FIG. 4, can be rotated in the mutually opposite directions. On the upstream side of the moving direction of the photosensitive body, in the developing area, there is disposed a first developing roller 61 which can be moved in the opposite direction to the moving direction of the photosensitive body; and, on the downstream side thereof, there is disposed a second developing roller 62 which can be moved in the same direction to the moving direction of the photosensitive body. That is, in the center feed developing system, since it is capable of both reverse-rotation development and forward-rotation development, the disadvantages of the respective developing systems can be compensated to thereby be able to obtain good image quality.
However, when either of the above-mentioned reverse-rotation or forward-rotation developing system including a developing roller is employed in high-speed printing, there arises the following problem: that is, the developing ability is scarce because of the increased rotation and, in order to compensate this, the number of rotations of the developing roller must be increased; and, therefore, there is caused a vicious circle that stresses to be given to the developing agent increase to thereby shorten the life of the developing agent and impair the stability of the quality of the image formed.
Also, the present inventors have conducted examination tests to check the center feed developing system for the performance thereof when it is employed in high-speed printing. According to our tests, we have found the following problems. That is, when compared with the developing system including a single roller, the center feed developing system is enhanced in the high developing performance and is able to reduce the number of rotations of the developing rollers. However, in the center feed developing system, the ratio between the peripheral speed of the first developing roller and the peripheral speed of the photosensitive body as well as the ratio between the peripheral speed of the second developing roller and the peripheral speed of the photosensitive body have great influences on the quality of the image formed. That is, in case where the respective peripheral speed ratios go below their optimum ranges, even in the case of the center feed developing system, the developing performance is lowered to thereby have an ill influence on the image quality. On the other hand, in case where the respective peripheral speed ratios exceed their optimum ranges, the developing performance can be enhanced but stresses applied to both of the toners and developing agent increase to thereby have an ill influence on the life of the developing agent and thus impair the stability of the image quality. As can be seen from this, even in the center feed developing system, in case where the respective peripheral speed ratios deviate from their optimum ranges, the image quality and the characteristic of the developing agent can be ill influenced.
Here, description will be given below in more detail of the case in which the respective peripheral speed ratios are set so as to exceed their optimum ranges. In the case of the center feed developing system, as shown in FIG. 4, between the first and second developing rollers, there is interposed a developing agent distributing member 8; and thus, the developing agent can be restrictively distributed to the respective developing rollers by the developing agent distributing member 8. In such restrictive distribution of the developing agent, however, large stresses are applied to both of the toners and developing agent. Due to this, there arise the following problems: that is, external additives added to the surfaces of the toners can be embedded into the toner surfaces, and the toner components can be fused (spent) to the surfaces of the carriers, with the result that the frictional electric charging between the toners and carriers can be insufficient, a sufficient amount of charged electricity cannot be obtained, the toners can be scattered, a photographic fog can increase, and the life of the developing agent can be lowered.